Abstract

Fatigue tests were conducted on 54 specimens of 25 ST aluminum alloy for the purpose of determining the effect of surface roughness on the fatigue life of the material when subjected to constant repeated tensile stresses above the proportional limit. In addition, the basic stress vs. cycle curve for 25 ST aluminum alloy was extended to include the range of cycles below 100,000.

A machine capable of applying repeated pure tension loads at a rate of 52 cycles per minute, without shock but with a high rate of loading, was used to obtain this data.

It was found that the rate of build-up and the duration of the impulse created an equivalent static load equal to the peak of the impulse loading.

For the material tested, it was found that as the surface roughness increased from 5μ to 200μ, the life expectancy of the alloy in general was reduced. However, the experimental results revealed a larger degree of scatter in the cyclic range below 40,000 cycles as opposed to the relatively consistent data obtained at the higher cycles. Therefore, no general conclusions could be ascertained as to the effect of roughness on the fatigue life of the material in the high stress region.